Aromatic carboxylic acids are important as fundamental chemical product and, in particular, aromatic dicarboxylic acids are useful for the starting material of fibers, resins and the like. For example, terephthalic acid has found its growing demand as the starting material for polyester fiber in recent years.
A process has hitherto been employed in general for producing an aromatic carboxylic acid, in which an aromatic compound having substituent alkyl group(s) is subjected to a liquid phase oxidation by bringing it into contact with a molecular oxygen-containing gas in a liquid reaction solvent comprising a lower aliphatic carboxylic acid, such as acetic acid, in the presence of an oxidation catalyst composed of heavy metal compound(s) and of bromine compound(s) in an oxidation reactor. According to such a production process, a mixture composed of an alkyl-substituted aromatic compound, such as paraxylene, as the starting material, acetic acid as the reaction solvent and a catalyst is supplied to the oxidation reactor while introducing thereinto a molecular oxigen-containing gas, such as air, to cause oxidation, whereby an aromatic carboxylic acid, such as terephthalic acid is obtained.
The exhaust gas from the oxidation reactor contains a vaporized portion of the reaction solvent and an entrained, or carried-over, portion of liquid droplets of the reaction mixture containing the catalyst. In order to recover and reuse them, a technique has been proposed, in which a distillation column is arranged so as to communicate to the top of the oxidation reactor and the distillation is carried out under utilization of the heat content of the oxidation exhaust gas so as to recover and reflux the solvent and catalyst to the oxidation reactor (See, for example, Japanese Patent publication Sho-54-14098 B and Japanese Patent Application Kokai Hei-6-279353 A). In this process, the distillation overhead gas is cooled in a condenser by a cooling water to condense steam and the so-condensed water is refluxed to the distillation column.
In order to attain a recovery of energy, it has also been proposed to incorporate in such an apparatus having a distillation column disposed directly above the oxidation reactor a technical measure of heating the distillation column overhead gas or occasionally even further burning it to drive a turbine (expander) (See WO 96/11899 and WO 97/27168). For energy recovery by a turbine, a higher temperature difference is required in general and, therefore, the above-mentioned technique does not employ a condenser subsequent to the distillation colmn so as not to lower the temperature but, on the contrary, employs heating of the overhead gas before it is guided to the turbine for attaining energy recovery.
Here, there occurs a problem that the turbine may be apt to suffer from obstructive phenomena of corrosion and scale formation, since the distillation colomn overhead gas contains, though in small amounts, corrosive ingredients, such as acetic acid etc., and scale-forming components. In addition, it is also problematic in the above-mentioned technique, that the mother liquor separated in the purification step is recycled to the distillation column, wherein the column may tend to suffer from clogging due to accumulation of the remaining carboxylic acid crystals.
On the other hand, there has been proposed a technique in which the exhaust gas from the oxidation reactor is directly guided to a condenser to form a condensate which is subjected to distillation to separate acetic acid, whereupon the so-separated acetic acid is returned to the oxidation reactor, while the condenser exit gas is passed to a layer of activated carbon to collect methyl acetate by adsorption thereon, which is desorbed thereafter by steaming the activated carbon layer and is then hydrolyzed to reproduce acetic acid to be recycled to the oxydation reactor (See Japanese Patent Application Kokai Hei-4-169551 A).
This technique requires, however, separation of each of the components for both the condensate and the condenser exit gas, since the oxidation reactor exhaust gas is subjected directly to condensation in the condenser, in addition to intricate operations for the absorption and desorption of the activated carbon as well as for the hydrolysis of methyl acetate desorbed.